1. Field of the Invention
The present invention relates to an ophthalmologic apparatus having a function of measuring the corneal thickness of an eye to be examined and a control method for the apparatus.
2. Description of the Related Art
In general, an ophthalmologic apparatus designed to measure the corneal thickness of an eye to be examined and the eye pressure performs alignment between the eye to be examined and the apparatus in the vertical direction, the lateral direction, and the operation distance direction (the forward/backward direction, that is, the direction to approach or separate from the eye) with respect to the eye.
A corneal thickness measurement apparatus disclosed in Japanese Patent No. 3597274 performs alignment by using a first projection/light receiving system which performs alignment in the vertical and lateral directions and a second projection/light receiving system which performs alignment in the operation distance direction and is also used for corneal thickness measurement. This apparatus performs alignment in the operation distance direction by using the reflected light of an alignment index projected from the outside of an optical axis facing the eye to be examined to the cornea. When performing alignment in the operation distance direction by using such a system, two projection/light receiving systems are required for alignment and corneal thickness measurement. This makes it impossible to simplify the optical system of the apparatus, and hence leads to increases in the size and cost of the apparatus.
In this case, according to an alignment scheme of a non-contact eye pressure meter disclosed in Japanese Patent Laid-Open No. 2006-334441, an apparatus includes one projection system (shared for alignment and corneal thickness measurement) and two light receiving systems (one of which is a light receiving system for measuring a corneal thickness, which is arranged at a position outside an optical axis facing the eye to be examined). This apparatus performs alignment in the vertical direction, the lateral direction, and the operation distance direction. In this case, a cornea bright spot image is an image that is formed when the cornea of the eye to be examined is illuminated with an alignment index and the reflected light is received from an optical axis facing the eye through a prism.
In this case, when performing alignment by using cornea bright spot images, the operation distance (the distance between the eye to be examined and the apparatus in the forward/backward direction) varies depending on not only alignment errors, but also on the differences in curvature between the surfaces of the corneas. For this reason, when measuring corneal thicknesses, the angle of the optical axis of the light receiving system that receives scattered light from the corneas and the optical path lengths to the light receiving element varies. As a consequence, blur occurs in scattered light from the cornea imaged on the light receiving element, resulting in a failure to perform accurate corneal thickness measurement.